Cathode sputtering refers to a process which involves the coating of a wafer mounted within a processing chamber. An applied electric field positively biases the wafer with respect to an oppositely mounted, negatively biased target made of the material to be sputtered. A backplate supports the target. Upon introduction of an inert gas into the chamber at low pressure, the applied electric field ionizes the gas. As a result, positive ions from the gas bombard the target to cause sputtering of the target material onto the wafer. A magnet or electromagnet located behind the target provides a magnetic field above the wafer facing surface of the target to confine the ion "plasma" adjacent to the target. The magnetic field increases the efficiency of the sputtering process.
Some typical materials used in sputtering include aluminum alloys, gold, silver, copper, titanium, platinum, titanium-tungsten, and refractory metal silicides. Due primarily to the high purity required for sputtering, the costs of these sputtering materials is generally quite expensive, on the order of about $100 per pound of material. For this reason, maximum target material utilization is a major consideration in the design of sputtering equipment. Another important consideration relates to the effective coverage of trenches or via features of the wafer, either by planarizing an uneven surface or by adding another step-like layer. Some sputtering equipment employs a variation of the magnetic field, or multiple, non-planar erosion zones, or both, to coat wafer features in the desired manner.
In addition to maximum target utilization and effective coverage of a wafer surface, other practical considerations such as target mounting and cost of manufacture also play an important role in the design of sputtering equipment. While some target shapes may provide excellent step coverage or planarization, these advantages may be offset by the difficulty or cost associated with properly mounting the target to a backplate. For the most part, the machining of targets is difficult, expensive and time consuming. However, as a general rule, simpler shapes are easier to produce.
It is an object of this invention to provide a cathode sputtering target with high target material utilization capability and which is relatively easy to mount to a backplate.
It is another object of the invention to provide a sputtering target assembly that promotes maximum target utilization and ease in mounting without producing a corresponding sacrifice in effective wafer coverage or excessive manufacturing costs.